1. Field of the Invention
This invention relates to catadioptric projection systems. More specifically, this invention relates to non-interfering viewing arrangements which enable two or more viewers to simultaneously view displays in such projection systems.
While the present invention is described herein with reference to a particular embodiment, it is understood that the invention is not limited thereto. Those having ordinary skill in the art and access to the teachings provided herein will recognize additional embodiments within the scope thereof.
2. Description of the Related Art
In conventional catadioptric aircraft simulators, a visual display is typically projected onto a reflecting screen on the inner surface of a dome. Certain simulators are designed to accommodate two or more viewers in order to allow, for example, fellow aircraft crew members to interact during simulator training. Unfortunately, projection systems used in simulator domes are generally operative to minimize the distortion of the projected image relative to a single preferred viewing position. Thus, essentially two possibilities exist with respect to positioning crew members within the simulator. First, one crew member may be placed in the preferred viewing position (i.e. at the focal point of the image). In this approach the display may appear relatively distorted to the viewer displaced from the preferred position. Second, the crew members may be symmetrically positioned with respect to the preferred viewing position. This configuration results in each crew member experiencing approximately equal degrees of display distortion.
A relatively expensive alternative to placing a pair of crew members within a single simulator dome is to position the crew members in separate simulators linked by a communication path. In this approach each crew member will usually be provided with a separate simulated cockpit, motion platform and visual display. However, this is an obviously somewhat more expensive approach.
The high cost of constructing separate simulators has led to the development of techniques for reducing the image distortion arising from the inclusion of more than one crew member under a particular simulator dome. In one such technique, often characterized as time multiplexing, the simulator projector rapidly alternates between projecting first and second displays within the simulator dome. The first display is intended to minimize the distortion associated with a viewing position occupied by a first crew member and the second display is generated so as to minimize the distortion experienced in viewing from the location of the second crew member. In this manner, each display is projected for approximately one-half of the length of the simulation. Each crew member wears a helmet having shutters (e.g. realized by liquid crystals) which appropriately open and close in synchronism with the alternating display.
Unfortunately, since each crew member is exposed to alternating light and dark image frames the effective bandwidth of the display is doubled--thereby degrading image contrast. Moreover, when time multiplexing is used in a simulator incorporating a single cathode ray tube (CRT) the switching time thereof may not be sufficiently rapid to ensure that crew members are isolated from viewing image frames intended to be seen by other crew members. That is, the phosphor screens of CRT's may "glow" from between a few microseconds to a few milliseconds subsequent to interruption of the electron beam impinging thereon.
A second technique for reducing image distortion within catadioptric simulators adapted to be used by more than a single occupant is often referred to as polarization multiplexing. In particular, a display intended for a first viewer is projected in a first polarization state while a display intended for a second viewer is projected in a second polarization state. Each viewer is equipped with a viewing helmet or the like operative to transmit light energy of one of the polarization states and to reject light polarized in the other. In this manner, each viewer only sees the display which has been projected in a manner designed to minimize the image distortion at the location of the viewer. However, the efficacy of this technique is impaired by the depolarization of the light energy included within each image upon reflection thereof by the simulator screen. This depolarization makes the "wrong" display partially visible to each viewer.
Accordingly, a need remains in the art for a viewing system disposed to simultaneously provide independent, non-interfering displays for two or more viewers.